Actuator assemblies are commonly used in aircraft. A screw actuator, for example, may be connected to an airframe at a motor end, e.g., by a spherical bearing or clevis bearing, and by a gimbal arrangement installed on a ballscrew nut of the actuator. The assembly may include a rod, which can act in tension or compression, that extends between a motor/gearbox housing and a portion of the airframe. The rod reacts torque experienced by the motor/gearbox during operation of the screw shaft. The nut on the screw shaft may be connected to a movable element, such as a flap or stabilizer, in particular a horizontal stabilizer. The stabilizer may be mounted to the airframe such that movement of the nut along the screw shaft operates the stabilizer.
No-back devices are commonly used in screw actuators like this to prevent feedback forces generated by loading the nut of the actuator, from feeding back into the actuator's motor. When a nut of a screw actuator is loaded by an external force (i.e. forces other than that from motor driving the screw shaft, such as forces created through air flow on a stabilizer) the interaction of the nut with the screw shaft will tend to rotate the screw shaft, as the nut tries to translate along the screw shaft in the direction of the external force. A no-back device may be provided on the screw shaft to prevent or at least minimise rotation of the screw shaft induced in this manner.
When external forces are applied to a nut, the nut will experience a force along the screw shaft, either towards the motor or away from the motor. The handedness of the screw shaft thread will determine whether a force towards the motor induces a clockwise or an anticlockwise torque. No-back devices commonly make use of this fact by providing two braking mechanisms—one braking mechanism will provide, say, clockwise braking forces when the screw shaft is loaded towards the motor/gearbox, and the other braking mechanism will provide anticlockwise braking forces when the shaft is pulled away from the motor by feedback forces. The braking forces that the no-back device can provide are aimed to be sufficient to counteract the maximum feedback torque applied to the screw shaft by the nut. However, no-back devices can fail, either gradually or suddenly.
There is a desire to be able to monitor the health of a no-back device. U.S. Pat. No. 8,918,291 B2, for example, discloses a method and system for monitoring an actuator equipped with a no-back device. The examples disclosed therein involve monitoring of the inputs and outputs of the motor (e.g. voltages for an electric motor or pressures for a hydraulic motor) in conjunction with rotary sensors on the output shaft to determine dysfunction of the no-back device.
It would also be desirable to improve the functionality of the actuator assembly through adaptions that do not affect the primary role of the components.